This invention relates to equipment for launching tools for servicing subsea oil wells.
During the drilling, testing and operation of an oil well it is sometimes necessary to insert and withdraw instruments such as well logging instruments, to deploy tools, e.g. "fishing" tools, and to replace equipment such as down-hole safety valves, pressure plugs, etc.
These operations are often carried out by the technique known as wirelining in which specially designed equipment is lowered down the well suspended from a solid or braided wire.
An alternative technique is that known as "pumpdown" in which, as the name implies, the tool or instrument is pumped through a line to the location where it is required, and returned after use. In theory, pumpdown offers advantages over wirelining, particularly for subsea completions. However, pumpdown has not been widely adopted, mainly because of the added complexity of the completion, the cost and complexity of the additional flowlines and the diverter systems required for multiwell template designs.
Wirelining and pumpdown are reasonably straightforward operations in onshore work where access to the well is readily available through a conventional "Christmas tree" well head at the earth surface.
Offshore production may be carried out from fixed platforms resting on the sea bed or from semi-submersible or floating platforms or vessels which are capable of some degree of movement.
Fixed platforms generally have several individual well risers rising from the sea bed to well head completion equipment on the platform and are analogous to on-shore locations in that access to the wells for wirelining or pumpdown operations is readily available.
In respect of semi-submersible and floating platforms and vessels, however, wells are generally completed on the sea bed and manifolded to a production riser system, or, in the case of satellite wells, may be remote from the production facility and tied back with flowlines and risers. Thus, immediate access to these wells from the surface is not normally available.
Access can be made available by fitting a tensioned riser back to the surface, but this is difficult, time consuming and expensive and may involve considerable loss of production. One method by which this can be achieved when the well is in close proximity to a movable production platform or vessel is to move the latter so that it is positioned with its moon pool or similar facility positioned directly above the well scheduled for wirelining. Another, which is more suitable for satellite wells, involves the temporary use of a drilling or workover vessel.
Both methods involve the use of a tensioned riser supported from the surface on which conventional surface equipment is mounted.